In a large deep skillet, heat oil over MED heat until it reaches approximately 350°F. Line a baking sheet with a paper towel, then top with a cooling rack. Set nearby, as this is where the chicken will rest once fried.
In a mixing bowl, combine flour, cornstarch, baking powder, baking soda and cold water and whisk until smooth. Set near your stove.
Season chicken pieces with salt and place near the mixing bowl.
Whisk sauce ingredients together in a small mixing bowl and set aside.
When oil is ready, dip chicken pieces, one a time, into batter and then carefully add them to the hot oil. Add about 10 pieces to the oil at a time.
Let fry for 2-3 minutes, or until golden brown. Remove with a metal spider or slotted spoon, and place on prepared baking sheet/cooling rack. Repeat with remaining chicken.
Drain out all but about 1-2 Tbsp of the oil and keep heat over MED HIGH. Add onion and bell peppers to the skillet and cook for about 2 minutes, until crisp tender.
Add garlic and pineapple and cook about 1 minute.
Pour sauce into skillet, stirring well to coat. Cook until sauce is bubbling and thickening, then add in fried chicken pieces and stir well.
Serve immediately, garnished with sesame seeds and sliced green onions if desired. We like to serve white rice alongside.